Artist’s rendition of a Hot Jupiter. Image: Hubble

“Hot Jupiters” aren’t particularly sexy exoplanets—just clingy ones. These gas giants orbit tightly around their host stars, and despite their name, they’re typically more massive than Jupiter. And, as you’d expect, much hotter.

According to the University of Arecibo, 74o of the 3,442 confirmed exoplanets are Hot Jupiters. These distant giants—full of gas and mystery—have certainly piqued our interest in recent years as exoplanet research has evolved. As Space.com notes, some hot Jupiters defy theoretical models of planetary formation because they’re so damn large. But new research suggests we might have the answer to that enigma, at least: Hot Jupiters aren’t born abnormally big—they just “puff” up over time.

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Using the Hungarian-made Automated Telescope Network (HATNet) in Mount Hopkins, Arizona and Mauna Kea, Hawaii, a team of scientists identified two particularly large Hot Jupiters, dubbed HAT-P-65b and HAT-P-66b. These exoplanets—2,745 and 3,025 lightyears from Earth, respectively—orbit their star 10 times closer than Mercury orbits our sun.

After comparing these two Hot Jupiters to 200 other exoplanets, the scientists found that HAT-P-65b and HAT-P-66b are unusually large for their respective ages (5.46 billion and 4.66 billion years-old). The team hypothesized that since these two Hot Jupiters orbit so closely to their host stars, they receive enormous amounts of radiation, over time expanding like cosmic pufferfish. The group’s findings have been published in the December issue of The Astronomical Journal.

Joel Hartman, a lead author on the study, suggests the research offers new insights about these elusive giants.

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“[We found] many [Hot Jupiters] are a lot larger than predicted by theoretical models of planetary structure (some planets are up twice the size of Jupiter, but the largest they could be, according to the models, is about 1.5 times the size of Jupiter),” he told Gizmodo. “Some of them are on wildly inclined orbits with respect to the spins of their hosts—some even orbit backwards around their stars.”

It’s no coincidence that HAT-P-65b and HAT-P-66b have ballooned to become 1.9 and 1.6 times Jupiter’s diameter. Hartman and his team found that both of the planets’ respective stars have completed 80 percent of their life cycles, meaning they’re nearing the end of the main sequence. Before they die, stars burn brighter and emit more radiation, which could cause their Hot Jupiters to expand.

“As stars get older they also get brighter, and deposit more energy into the upper atmospheres of any close-in planets they might harbor,” Hartman said. “If this energy can make its way down to the core of a gas giant planet it would cause the planet to expand. This idea has been floating around for quite a while as a possible explanation for why some hot Jupiters can be extremely large. But, no one has been able to convincingly demonstrate a mechanism for transporting the energy deep into the interior of the planet.”

This study is much more than unlocking the secrets of a cool space mystery, though it totally does that, too. The research helps us better understand how a star’s radiation can impact the way planets evolve.

“Furthermore, if we want to apply these theoretical models to infer the properties of smaller and more distantly orbiting planets which are harder to observe in detail (e.g., habitable Earth-like planets), we need to test the theories, and make sure they work for the planets that we can study in the most detail,” Hartman said.

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[The Astronomical Journal]